Polyethylene film is used in a wide variety of applications such as the preparation of food packages, heavy duty sacks, collation shrink packaging and trash can liners. Polyethylene is commonly described in terms of melt index (which provides an indication of the molecular weight of the polyethylene) and density.
The MDO films described in this disclosure are prepared with a precursor film (i.e., a non-stretched film) that contains at least one layer of HDPE and at least one layer that is made from a polyethylene having a lower density.
HDPE is now commonly used in the preparation of “barrier film” which is resistant to the transmission of water. Barrier film is especially suitable for packaging dry foods such as breakfast cereals and crackers. Monolayer HDPE barrier film has limited use because several of the properties of such film—including impact strength, tear strength and sealing temperature—are inferior to those of lower density polyethylene. The use of a multilayer film that contains a layer of HDPE and a layer of a lower density resin can mitigate some of these problems but this is generally done at a cost of reducing the barrier properties of the multilayer film in comparison to a monolayer HDPE film.
This disclosure provides a multilayer film that is subjected to machine direction orientation (MDO) in a process whereby the film is stretched after the film is initially formed in a conventional extrusion process. One layer of the film is made from a high density polyethylene composition which, in one embodiment, contains a nucleating agent. The multilayer MDO films of this disclosure exhibit a balance of barrier and physical properties that makes them suitable for many packaging applications.